Boise Best & Worst AQI Days: Air Quality Guide
The worst air quality day on record for Boise (Boise City, ID area) was September 1, 2024, when the daily AQI reached 265 (Very Unhealthy), set by fine particulate matter (PM2.5), in wildfire smoke season. Across 2,557 monitored days in 2019–2025, 10 of the 10 worst days were Unhealthy (AQI 151+) or worse; 9 of the 10 were driven by fine particulate matter (PM2.5) and 1 by ground-level ozone, and 8 of them fell in 2024. The cleanest day was May 20, 2019 at AQI 12 (Good); the 10 best days span 4 different years.
Reduce Indoor Ozone in Boise
Today's main pollutant in Boise is ground-level ozone (AQI 38). Important: standard HEPA filters do NOT remove ozone — only a purifier with a substantial activated-carbon bed adsorbs it. The biggest wins are cutting indoor sources (gas appliances, certain "ionizer" purifiers) and ventilating when outdoor ozone is low (early morning). A high-carbon unit helps with the gases and VOCs that linger indoors.
Recommended: Levoit Core 300 — True HEPA + activated carbon · CADR ~141 CFM · rooms up to 219 sq ft
As an Amazon Associate, we earn from qualifying purchases.
When were Boise's worst and best air quality days?
Worst: September 1, 2024, AQI 265 (Very Unhealthy, PM2.5). Best: May 20, 2019, AQI 12 (Good). Based on 2,557 EPA-monitored days in the Boise City, ID area, 2019–2025.
- ✓10 of the 10 worst days were Unhealthy (AQI 151+) or worse; the 10th-worst was AQI 188 (Unhealthy).
- ✓Fine particulate matter (PM2.5) drove 9 of the 10 worst days.
- ✓2024 accounts for 8 of the 10 worst days.
Frequently asked questions
What was the worst air quality day in Boise?▾
September 1, 2024: the Boise City, ID area's daily AQI hit 265 (Very Unhealthy), driven by fine particulate matter (PM2.5), in wildfire smoke season. That is the highest of 2,557 monitored days in 2019–2025.
What was the best air quality day in Boise?▾
May 20, 2019, with a daily AQI of 12 (Good). The daily AQI is the highest reading from any monitor in the area, so even the cleanest day reflects the area's worst monitor.
What was Boise's worst air quality day in 2025?▾
December 11, 2025, AQI 135 (Unhealthy for Sensitive Groups), set by fine particulate matter (PM2.5).
Which pollutant causes Boise's worst air days?▾
Fine particulate matter (PM2.5) set the AQI on 9 of the 10 worst days; ground-level ozone on 1. The tables above list the defining pollutant for every day.
Boise's 10 worst air quality days, 2019–2025
Highest daily AQI recorded for the Boise City, ID area (EPA AQS). The daily AQI is the highest reading from any monitor in the area that day.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2024 page) | 265 | Very Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 2 | (2024 page) | 260 | Very Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 3 | (2024 page) | 251 | Very Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 4 | (2020 page) | 223 | Very Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 5 | (2024 page) | 214 | Very Unhealthy | O3 — ground-level ozone, in peak ozone season |
| 6 | (2024 page) | 194 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 7 | (2024 page) | 191 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 8 | (2024 page) | 191 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 9 | (2024 page) | 189 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 10 | (2022 page) | 188 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
Worst day of each year in Boise
The single highest-AQI day in each year on record. Click a year for its full month-by-month statistics.
| Year | Worst day | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 2019 | 170 | Unhealthy | PM2.5 — fine particulate matter (PM2.5) | |
| 2020 | 223 | Very Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2021 | 173 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2022 | 188 | Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2023 | 120 | Unhealthy for Sensitive Groups | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2024 | 265 | Very Unhealthy | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2025 | 135 | Unhealthy for Sensitive Groups | PM2.5 — fine particulate matter (PM2.5) |
Boise's 10 best air quality days, 2019–2025
Lowest daily AQI recorded for the Boise City, ID area. Ties are listed most recent first.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2019 page) | 12 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 2 | (2023 page) | 18 | Good | NO2 — nitrogen dioxide |
| 3 | (2025 page) | 22 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 4 | (2025 page) | 22 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 5 | (2025 page) | 23 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 6 | (2024 page) | 23 | Good | NO2 — nitrogen dioxide |
| 7 | (2024 page) | 23 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 8 | (2025 page) | 25 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 9 | (2024 page) | 25 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 10 | (2019 page) | 25 | Good | O3 — ground-level ozone |
Right now in Boise
The latest EPA AirNow observation puts Boise's AQI at 38 (O3). The days above are historical records, not today's reading.
View current Boise air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (Boise City, ID), 2019–2025. Daily AQI per CBSA is the highest AQI reported by any monitor in the area that day; the defining parameter is the pollutant that set it. Month and season statistics pool every such day in the period across all years listed. Retrieved 2026-09-09.